Why can't I install third party apps

I have just bought a galaxy s7 edge running andriod marshmellow and I am unable to install ANY third party apps (tried a few), even though I've enabled it in settings. Either nothing happens when I click on the downloaded file or an advert pops up for various different apps which then directs me to app store.

Welcome to the forums. You have a Gmail account, correct? Does this happen over both your cell signal and wifi?

Hi and thanks for the welcome. Yes I have a Gmail account ndiyo I've had the same problem over cell and WiFi signals.

Welcome to Android Central! Are you installing through Google Play Store, or some other app store? Or are you downloading apk files from some random site?